1. Field of the Invention
The present invention relates to a method for forming a silicon semiconductor thin film which has a crystallinity and is formed on a substrate having an insulation surface of a glass substrate or the like.
2. Description of the Related Art
In recent years, attention is paid on a technology for constructing a thin film transistor by using a silicon thin film which is formed on a glass substrate. This thin film transistor is primarily used in an active matrix type liquid crystal electro-optical device and other thin film integrated circuits. The liquid crystal electro-optical device changes optical characteristics of a liquid crystal thereby displaying an image by charging a liquid crystal into a pair of glass substrates and applying an electric field.
In particular, the active matrix type liquid crystal display device using a thin film transistor is characterized in that the thin film transistor is arranged in each pixel, an electric charge held in a pixel electrode is controlled by using the thin film transistor as a switch. Since the active matrix type liquid crystal display device is capable of displaying a fine image at a high speed, the device can be used in displays for various electronic apparatuses (for example, a portable word processor, and a portable computer or the like).
As a thin film transistor used in the active matrix type liquid crystal display device, an amorphous silicon thin film is commonly used. However, the thin film transistor using the amorphous silicon thin film has the following problems.    (1) The liquid crystal thin film transistor has low characteristics and cannot display a higher quality image.    (2) The liquid crystal thin film transistor cannot constitute a peripheral circuit for driving a thin film transistor arranged on a pixel.
The aforementioned second problem can be considered by dividing the problem into the following two aspects. One aspect of the problem is that since a P-channel type thin film transistor cannot be used for practical purposes with the thin film transistor using an amorphous silicon thin film, a CMOS circuit cannot be constituted. Another aspect of the problem is that since the thin film transistor using an amorphous silicon thin film cannot be operated at a high speed, and a large current cannot flow in the thin film transistor, a peripheral driving circuit cannot be assembled.
Means for solving the aforementioned problems include a technology for forming a thin film transistor by using a crystalline silicon thin film. The methods for obtaining a crystalline thin film include a method for heat treating an amorphous silicon film and a method for irradiating the amorphous silicon thin film with laser light.